The discrete anatomical distribution of arginine vasopressin and corticotropin releasing factor (CRF) immunoreactivity in the paraventricular nucleus (PVN) of the rat hypothalamus is altered after adrenalectomy. Not only is the immunostaining of both peptides enhanced, but vasopressin immunoreactivity, normally confined to the magnocellular subdivision, becomes clear in a large percentage of CRF neurones in the parvocellular subdivision. These changes in immunoreactivity may reflect changes in post-translational events, peptide metabolism or genomic activity that lead indirectly or directly to the enhanced expression of vasopressin. Here we report that levels of transcripts homologous to vasopressin messenger RNA increase in the PVN after adrenalectomy, in parallel with increases in vasopressin immunoreactivity. In fact, after adrenalectomy, vasopressin mRNA can be detected in CRF-immunoreactive neurones. These results indicate that a considerable degree of plasticity is retained by the adult neuronal genome of the rat and that this plasticity may be modulated by the endocrine environment.

Immunocytochemical studies have shown that adrenalectomy produces changes in the content and distribution of [arginine-8]vasopressin (AVP) immunoreactivity in the paraventricular nucleus of the hypothalamus. The purpose of this study was to determine whether manipulation of adrenal hormones affects the levels of AVP mRNA. In situ hybridization assays with highly specific synthetic oligodeoxyribonucleotide probes and immunocytochemistry were used to detect the distribution of AVP mRNA and AVP-immunoreactive perikarya. AVP mRNA is codistributed with AVP immunoreactivity in the posterior magnocellular subdivision of the paraventricular nucleus and its accessory nuclei, the supraoptic nucleus and the suprachiasmatic nucleus. In adrenalectomized rats, the density and distribution of the hybridization signal were increased in the paraventricular nucleus; a 2-fold increase in the area comprising the signal was observed. At the cellular level, silver grains were detected in corticotropin-releasing-factor-immunoreactive neurons throughout the medial parvocellular subdivision of the paraventricular nucleus. No changes were seen in the distribution of AVP mRNA in the supraoptic or suprachiasmatic nuclei. Treatment with dexamethasone prevented the increase in AVP mRNA produced by adrenalectomy. In contrast, adrenalectomy did not alter the hybridization signal obtained with a probe for a-tubulin mRNA. These results suggest, at the cellular level, that adrenalectomy induces a glucocorticoid-sensitive stimulation of AVP mRNA synthesis in the central nervous system. Thus, considerable plasticity in gene expression is retained in the hypothalamus of the adult rat.The discrete anatomical organization of hypothalamic neurosecretory neurons within the paraventricular nucleus (PVN) has established this region as a unique site to study the neurohypophyseal secretory system. [arginine-8]Vasopressin (AVP) and oxytocin are contained within neurons of the PVN and the supraoptic nucleus (SON) (1, 2) and are involved in both autonomic and neurosecretory functions (3, 4). The axons emanating from these perikarya form the hypothalamoneurohypophyseal tracts, which project to the pars nervosa where AVP is released from their terminal endings (5,6). However, AVP has been detected in axon terminals within the external zone of the median eminence (7), suggesting that it may be involved, to some extent, in regulating anterior pituitary functions. Unlike the neurohypophyseal projection, the majority of AVP-immunoreactive axon terminals in the median eminence originate from neurons within the medial parvocellular subdivision of the PVN (8). Biochemical studies have confirmed quantitatively that adrenalectomy increases AVP immunoreactivity (7, 9). After adrenalectomy, corticotropin-releasing factor (CRF) and AVP were reported to be colocalized within neurons in the medial parvocellular region of the PVN (10, 12). This finding is of particular interest because immunocytochemical studies have shown that adrenalectomy produces a dexamethasonesen...

Previous studies have shown that adrenalectomy augments arginine vasopressin (AVP) messenger RNA levels in the adult paraventricular nucleus. It is now demonstrated that unilateral lesions in the lateral septal nucleus enhance the adrenalectomy-induced expression of AVP mRNA. This effect was entirely ipsilateral to the lesion and most prominent in the rostral paraventricular nucleus and related nuclei. Moreover, AVP and AVP mRNA were found to be colocalized with oxytocin in a few neurons. These results indicate that mRNA expression is modulated by synaptic influences and raise the possibility that synaptically mediated selection of neuronal phenotypes is a dynamic feature of the mature central nervous system.

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